1. Field of the Invention
The present invention relates to magnetic resonance (MR) spectroscopy, as applied in radiological diagnostics in the form of in vivo magnetic resonance spectroscopy (MRS) for the assessment of metabolic compounds in the human body. More particularly, the present invention relates to modifications of magnetic resonance experiments and also to an apparatus for execution of these experiments, to transfer polarization between nuclei with the aim to enhance the detection sensitivity of compounds by magnetic resonance or to enable spectral editing of molecular fragments based on their coupling to specific isotopes.
2. Description of the Prior Art
Magnetic resonance spectroscopy (MRS) like magnetic resonance imaging (MRI) is based on the nuclear magnetic resonance effect which was discovered in 1946 and in the very beginning used for examination of the magnetic characteristics of nuclei. Later it was found that the resonance signal of a nuclei is also influenced by its chemical surrounding and that this so-called chemical shift can be used to characterize chemical substances. This kind of examination was established as the so-called “high resolution NMR” in vitro. This high resolution NMR finds its application in the physical, chemical, biochemical and pharmaceutical research and development in order to analyze the structure of complex macro molecules.
In the late seventies of the 20th century it was newly discovered that the resonance signal can be used for non-invasive imaging of living species, and such imaging has become one of the most important radiological examination methods in medicine.
However, it was not ignored that magnetic resonance imaging further contains chemical information, which can be used to analyze biochemical reactions, particularly metabolism in vivo. This kind of NMR spectroscopy with spatial resolution related to the in vivo organism or related to live organs was called “in vivo spectroscopy” or “clinical nuclear magnetic resonance spectroscopy (MRS)” in contrast to “high resolution NMR” in the test tube, which is usually carried out in the laboratory, respectively in contrast to the mere magnetic resonance imaging (MRI).
In the following, the physical basics of nuclear magnetic resonance will be explained:
In MRS as well as in MRI the object to be examined (the patient or the organ) is exposed to a strong, constant magnetic field. Thereby, the nuclear spins of the atoms in the object, which were oriented randomly before, are aligned, building discrete energy levels. Radio frequency energy can cause transitions between these energy levels. If a radio frequency pulse e.g. enables a steady state population of the levels, an induced signal can be obtained after the switch off of the radio frequency field. Because of application of inhomogeneous magnetic fields initiated by so-called gradient coils, the object to be investigated can be excited selectively and the signals can be spatially encoded.
In MRS the data sampling is usually done in the so-called time domain, the sampling of the MRI data in the so-called k-space (frequency space). The MR spectrum in the frequency domain, namely the MRI image in the so-called imaging space, is correlated with the sampled data by Fourier-transformation. A volume excitation in the object is achieved in the object by slice selective radio frequency pulses, namely by simultaneous application of gradient pulses. For the excitation of a cuboid e.g. three slice selective high frequency pulses in three orthogonal directions are applied in the MRS. Normally, these are three Sinc-shaped, Gaussian-shaped or hyperbolic-shaped RF pulses, which are irradiated simultaneously with rectangular or trapezoid gradient pulses in the object to be examined. The irradiation of the RF pulses has to be effected by an RF antenna.
By the combination of the pulses a frequency spectrum in the range of a specific nuclear resonance frequency is radiated in a well-defined cuboid-shaped area of the object to be examined. The respective nuclei in this selected range (volume of interest, VOI) react on their part with electromagnetic response signals (electromotive force EMF), which are detected in the form of a sum signal (free induction decay signal FID-signal), respectively in form of a (half) spin echo signal by a special receiving condition of said RF antennas. This analogue signal (FID or echo) is sampled by switching of an ADC (analog-digital-converter), digitalized and saved on a computational apparatus, respectively Fourier-transformed, whereby a so-called “spectrum” can be displayed on a visualization apparatus (monitor).
Both components of the measured signals (FID or echo signals) characterize the projections of said oscillation behavior of the nuclear magnetization vector M in the x-y-plane of a stationary frame of reference (laboratory system of coordinates).
The temporary decay of the signal is determined by the T2-weighted transversal relaxation. The transversal relaxation leads to the disappearance of the time-dependent transversal magnetization Mxy(t), whereas the T2-time, more particularly the T2*-time, which considers local B0-field inhomogeneities ΔB0 according to the equation
                                          1                          T              2              *                                =                                    1                              T                2                                      +                          γ              ⁢                                                          ⁢              Δ              ⁢                                                          ⁢                              B                0                                                    ,                            (        1        )            as a characteristic time constant is determining the decay of the FID or echo signal. In the above equation, γ represents the gyromagnetic ratio, which describes the energetic coupling constant of the respective nuclei to the external magnetic field and which is a fixed constant of the respective nucleic species
The complex and time-dependent (therefore three-dimensional) FID or echo signal itself can be considered as the electromagnetic response to one or more circular high frequency exciting pulses, which have been irradiated into the respective substance or the tissue to be examined before.
In the case of the substance or tissue consisting of only one specific nucleic species (e.g. protons in pure water) and the RF excitation pulse being irradiated with a frequency, which corresponds exactly to the Lamor frequency of the protons (63,8 MHz at 1,5 Tesla), the measured FID respectively echo signal of the water protons will not contain any harmonic/periodic parts (sinus- or cosinus-shaped components), as in the (at 63,8 MHz) rotating referential system a precession/rotation of the transversal magnetization does not take place. (The relative movement in the rotation direction equals zero). Only the relaxation-dependent exponential reduction of the transversal magnetization vector is measurable, which constitutes a non-modulated exponential function (dashed line in FIG. 2A).
If the irradiated RF excitation pulse shows a frequency, which does not exactly corresponds to the water protons (e.g. 63.8 MHZ+400 Hz), but provokes an excitation of the protons due to its pulse amplitude anyhow, the measured FID respectively echo signal, at a referential frequency for the data acquisition equaling the frequency of the RF pulse, will contain a harmonic part of 400 Hz, which is—according to FIG. 2A—modulated to the exponential relaxation decay
      ⅇ                  -        t                    T        2        *              .
In general, the substance respectively the object to be examined (in the medical in vivo spectroscopy) will firstly not contain only one nucleic species (1H, 31P, 13C), but a plurality of nucleic species to be analyzed. Secondly, the nuclei of the same species will show relatively to each other different resonances (Lamor frequencies) due to their different integration into different molecules (different chemical environment) and can be distinguished as so-called metabolites.
In the (in vivo) proton spectroscopy the frequency range of most metabolite signals is about 10 ppm, the spectral width in the (in vivo) phosphor spectroscopy is ca. 30 ppm and in (in vivo) 13C-spectroscopy the resonances in the spectra are spread over an area of about 200 ppm. The indication of the changing of the resonance frequency δ relative to the system frequency (RF center frequency v0) in ppm (parts per million), i.e. in millionth of the resonance frequency according to the equation
                    δ        =                                                            v                substance                            -                              v                0                                                    v              0                                ·                      10            6                                              (        4        )            is advantageously independent of the strength of the magnetic field.
In general, the FID or echo signal forms a temporally dependent response signal—a so-called “signal imaging/representation in the time domain”—in whose exponential process all resonances (ωx, x∈N) of the excited nuclei in the respective metabolites are modulated and are superimposed and frequency-encoded.
An FID, which according to FIG. 2A contains the frequency response of only one metabolite, delivers according to FIG. 2B only one resonance line.
An FID, which contains e.g. the frequency responses of three different metabolites, is shown in FIG. 3A. It can be seen, that the FID respectively echo signal in FIG. 3A is encoded considerably more complex than the FID respectively echo signal of FIG. 2A, which shows only one frequency. This encoding can by decrypted by a Fourier-transformation and ordered by the respective resonance frequencies, whereby according to FIG. 3B a three-component spectrum with so-called resonance lines at ω0, ω1, and ω2 is obtained.
The Fourier transform of the FID respectively echo signals (FIGS. 2B, 3B) is generally referred to as spectrum. It is also referred to as “signal imaging/representation in the frequency domain”.
Although, as already mentioned, the gyromagnetic ratio γ (equation (1)) is a fixed constant of the respective nucleic species (e.g. for the proton is γ/2π=42, 577 MHz/T), in the same (constant) outer magnetic field slightly different resonance frequencies can be seen in NMR experiments, in which the examined nuclei are integrated in different molecules. Responsible for this are the electrons in the molecule, which cause the chemical binding. They shield the outer (external) magnetic field, so that the nuclei depending on the state of binding “sees” different magnetic fields (BK), which causes the already mentioned slight displacement of the respective resonance frequency and is referred to as “chemical shift δK”:Bk=B0−δKB0  (5)
In a molecular complex there is often a number of resonance lines, which can be assigned to single molecule groups. Quantitatively, according to equation (4) the chemical shift is mostly given in ppm relative to a reference line (v0).
Apart from the chemical shift, also a fine splitting of the nuclei resonance lines in form of multiplet lines (doublets, triplets, quartets, etc.) can be seen often. Responsible for this is the magnetic interaction (spin-spin-coupling) among the nuclei, which is not arranged over the space, but indirectly over the electrons of the chemical binding. For the analysis of the spectra with a fine structure usually the energy function (Hamilton operator Ĥ) with the interaction energy Jkl (scalar energy coupling constant) between the different spin states
                    J        _            ^        k    ⁢          ⁢  and  ⁢          ⁢                                              ⁢                  J          _                    ^        l  
                              H          ^                =                              -                          ∑                              γ                ⁢                                                                  ⁢                ℏ                ⁢                                                                  ⁢                                                      B                    0                                    ⁡                                      (                                          1                      -                                              δ                        k                                                              )                                                  ⁢                                                      J                    ^                                    zk                                                              +                                    ∑                              k                ,                l                                      ⁢                                          J                kl                            ⁢                                                                    J                    _                                    ^                                k                            ⁢                                                                    J                    _                                    ^                                l                                                                        (        6        )            whose eigenvalues and eigenfunctions describe the measured spectrum corresponding to the assumed molecular model. In this way, the structural clarification of (macro-) molecules is advantageously realized in chemistry and biochemistry. In the medical sector typical metabolites can be detected non-invasively on the basis of their spectra in vivo.
In imaging the low sensitivity of magnetic resonance using the proton signals of water is not a major issue as 1H nuclei have a large magnetic moment and water is abundantly present in the body. In MR spectroscopy, however, usually compounds at much lower tissue concentrations are observed and often also many MR nuclei with interesting physiologic information (e.g. 31P, 13C, 15N) are less sensitive than the 1H nucleus. A known class of methods for detection improvement in NMR-spectroscopy is known as polarization transfer, in which the high population difference of two or more energy levels of a particular nuclear spin system is transferred to the energy levels of a less populated other nuclear spin system by spin spin-coupling.
The principle of the polarization transfer-based detection improvement is explained in detail in the following:
Simplifying, the example contains a two-spin-system consisting of each one sensitive and one insensitive (slightly sensitive) nuclei, e.g. 1H and 13C.
In a magnetic field B0 such nuclei (spin quantum number ½) are able to adopt each to two discrete energy states. The transition between energy levels involves the absorption or emission of an electronic quantumω=ΔE=γB0  (7)
The allocation/population of the energy levels in the external magnetic field B0 takes place according to the Boltzmann-statistic
                                          N            q                                N            p                          =                              ⅇ                                          Δ                ⁢                                                                  ⁢                E                            kT                                ≈                      1            -                                          γ                ⁢                                                                  ⁢                ℏ                ⁢                                                                  ⁢                                  B                  0                                                            k                ⁢                                                                  ⁢                T                                                                        (        8        )            
Resulting therefrom is an excess of nuclei-magnetic moments parallelly aligned to the magnetic field B0.
Decisive for the population difference between two states Eq and Ep is the gyromagnetic relation γ of the respective nuclei, which changes its spin-adjustment/orientation during the transfer from Ep→Eq. For states, which belong to the transfers of a sensitive nucleic species A (high γ), a greater population difference results than for states, which belong to the transfers of an insensitive nucleic species X (low γ).
The population in the term scheme of such an AX-system consisting of a strongly sensitive nuclei (A) and a slightly sensitive nuclei (X) is schematically shown in FIGS. 4A, 4B and 4C.
FIG. 4A shows the state of equilibrium, in which the two lowest energy levels (1) and (2) are populated the most (symbolized by bold bars).
If an interchanging of the respective spin populations is achieved by a (selective) population inversion for an A-line (A1 or A2) in the NMR-spectrum, so the term scheme of FIG. 4B which now shows fortified absorption (X1) and fortified emission (X2) for the X-transfers, respectively the term scheme of FIG. 4C whereby X1 shows fortified emission and X2 fortified absorption, becomes valid. In both cases (FIGS. 4B, 4C) the population balance is perturbed by selective population inversion between the states (1) and (3) respectively between the states (2) and (4).
The population difference corresponding to the signal intensity, which was beforehand decisive for the sensitive nuclei, is now valid for the insensitive nuclei. This phenomenon is referred to as polarization transfer, which is used for enforcing the signal of NMR-insensitive nucleic species (X).
Of general interest is thereby the sensitivity improvement of 1H-coupled spectra of insensitive nuclei, as e.g. 13C (but also 15N or 31P), i.e. the intensity increasement of XAn-spin systems with A=1H and X=13C.
The diagram of an energy level of a CH-spin system (n=1) with different coupling is shown in FIGS. 5A, 5B and 5C.
FIG. 5A shows the four energy levels 1, 2, 3 and 4 without coupling onto the external magnetic field B0, which are possible due to different C-H-spin modulations, i.e. for the scalar energy-coupling-constant J=O. As in this case the 1H-transfers 3→1 and 4→2, respectively the 13C-transfers 2→1 and 4→3 are energetically equal, only one 1H-line and one 13C-line are resulting in the spectrum (no splitting or no hyperfine structure).
In FIGS. 5B and 5C a different example is shown, in which an energetic coupling of the C-H-spin states takes place, whereby in case of FIG. 5B the energy levels of the parallel spin states (↑↑,↓↓) are increased by J/4 and the energy levels of the antiparallel spin states (↑↓,↓↑) are decreased by J/4. In FIG. 5C an exactly inverse example is shown. The coupling γ1H≈4γ13C leads in every case to each two energetically different transfers of the respective atomic nucleic species, which leads to a double fine structure splitting in the spectrum, i.e. to two directly neighboring spectral lines in form of a doublet. Each nucleic species alone thereby experiences an overall energy changing of J.
To calculate the population relations (relative population-respectively transfer probability) for the polarization transfer and which is thereby relevant for the signal enhancement to be effected, it is advantageous, to regard the term scheme of FIGS. 4A and 4C more closely, i.e. quantitatively (see FIGS. 6A, 6B, 6C).
In FIG. 6A the lowest energy level features an energy of
            1      2        ⁢          γ      H        +            1      2        ⁢          γ      C      which is proportional to the population probability), while the other energy levels feature energies respectively population probabilities of
                    1        2            ⁢              γ        H              -                  1        2            ⁢              γ        C              ,                    -                  1          2                    ⁢              γ        H              +                  1        2            ⁢              γ        C            ⁢                          ⁢      and      ⁢                          ⁢      in      ⁢                          ⁢              1        2            ⁢              γ        H              -                  1        2            ⁢              γ        C            ascending order, corresponding to the respective coupled spin states (αα=↑↑=parallel to B0), (αβ=↑↓), (βα=↓↑), (ββ=↓↓=antiparallel to B0).
After a suitable (spin) preparation of the system by irradiation of suitable electromagnetic high frequency pulses in the context of a defined pulse sequence, energy can be added selectively to the system, in a way that the αβ-coupling changes into the energetically higher ββ-coupling. After the preparation, the system has spin-spin-pairs, which are parallel (αα=↑↑) and antiparallel (ββ=↓↓) to the magnetic field B0.
If, for clarity, one adds to the energy levels the constant energy amount of
                    1        2            ⁢              γ        H              +                  1        2            ⁢              γ        C              ,the energy states γH+γC, γH, γC and 0 are obtained. If the relation of the nucleic sensitivities of 1H and 13C (γH=4 and γC=1) is taken into account, the relative values of 5, 4, 1 and 0 are obtained for the energy levels according to FIG. 6B. These values correspond as well, as already mentioned, to the relative population probabilities respectively the relative populations, as the magnetic moment μ characterizing the sensitivity defines the differences of the energy levels as well as the population probabilities (according to Boltzmann).
As can be seen clearly in FIG. 6B, the population difference of the 13C-transfers is relatively low (Δ=1−0=+1; Δ=5−4=+1) in the unexcited system. According to that the 13C-doublet features a low NMR-signal intensity compared to the 1H-doublet. If the system, however, is forced into a higher-energy state by energy transfer (alignment of the spin-pairs antiparallel to B0), population differences of 13C-transfers result, which lead to an emission enhancement of Δ=1−4=−3 as well as an absorption enhancement of Δ=5−0=+5 in the spectrum (FIG. 6C).
This signal enhancement of an X-doublet in the MR-spectrum (e.g. X=13C) is shown in FIG. 7A. The unit of the ordinates was chosen randomly. Important is the significant enhancement of the two X-doublet lines.
The enhancement to a 3-atomic AX-spin system (e.g. to a CH2-group) leads to a significantly more complex term scheme of the energy levels and—as can be shown—to an X-triplet with the relevant intensities (1)-(2)-(1) in the spectrum (FIG. 7B). A signal enhancement leads to values of (−7)-(2)-(9) in this system.
The intensity enhancement, which is obtained in the general enhancement to AnX-spin systems (A=1H, X=13C), can be calculated by comparison with Pascal's Triangle according to FIGS. 8A and 8B.
Line numbers and relative intensities for an X-multiplet of an AnX-group (A=1H) at Boltzmann-distribution (FIG. 8A) and after population inversion (FIG. 8B) are shown. The respective triangle is obtained by combining the (integral) energy level transfers of the underlying term scheme.
The preparation of the spin system and thereby the realization of the polarization transfer can be achieved by using different RF pulse sequences. Most common is the INEPT-method (Insensitive Nuclei Enhanced by Polarization Transfer, Morris, Freeman, J. Am. Chem. Soc. 101, 760-762 (1979)).
Further methods are e.g. Refocused-INEPT, DEPT (Distortionless Enhancement by Polarization Transfer), SINEPT, etc.
Generally, all these methods are based—as will be explained later in more detail—on the concurrent appliance (irradiation) of RF pulses on the different frequencies of the participating nucleic species (i.e. for example 1H, 13C). This results in the disadvantage, that NMR apparatuses, which are not able to concurrently send in the different frequencies of the participating nuclei, are also not able to execute NMR experiments with polarization transfer.